Process for the preparation of precipitated silica, new precipitated silicas containing zinc and their use for the reinforcement of elastomers

ABSTRACT

The invention relates to a new process for the preparation of precipitated silica which has a good dispersibility and very satisfactory reinforcing properties. It also relates to new precipitated silicas which are in the form of powder, of substantially spherical beads or of granules, these silicas being characterized by the fact that they have a CTAB specific surface of between 90 and 250 m 2  /g, a DOP oil uptake lower than 300 ml/100 g, a pore distribution such that the pore volume consisting of the pores whose diameter is between 175 and 275 Å represents less than 50% of the pore volume consisting of the pores of diameters smaller than or equal to 400 Å, a zinc content of between 1 and 5% by weight and by the fact that the number N of molecules of stearic acid consumed per nm 2  of silica surface, when stearic acid is reacted with the said silica in xylene for 2 hours at 120° C., is at least 1. The invention also relates to the use of the said silicas as reinforcing fillers for elastomers especially for improving their rheological properties.

This application is a division of U.S. application Ser. No. 08/737,975,filed on Mar. 3, 1997.

The present invention relates to a new process for the preparation ofprecipitated silica, to precipitated silicas which are in particular inthe form of powder, of substantially spherical beads or of granules, andto their application as a reinforcing filler for elastomers.

It is known that precipitated silica has been employed for a long timeas a white reinforcing filler in elastomers.

However, like any reinforcing filler, it is appropriate that it shouldbe capable of, on the one hand, being handled and above all, on theother hand, of being easily incorporated into the mixtures.

It is known in general that, to obtain the optimum reinforcingproperties conferred by a filler, it is appropriate that the lattershould be present in the elastomer matrix in a final form which is bothas finely divided as possible and distributed as homogeneously aspossible. However, such conditions can be achieved only insofar as, onthe one hand, the filler has a very good ability to be incorporated intothe matrix during mixing with the elastomer (incorporability of thefiller) and to disintegrate or to deagglomerate into the form of a veryfine powder (disintegration of the filler) and as, on th other hand, thepowder resulting from the abovementioned disintegration process canitself, in its turn, be perfectly and homogeneously dispersed in theelastomer (dispersion of the powder).

Moreover, for reasons of mutual affinities, silica particles have anunfortunate tendency, in the elastomer matrix to agglomerate with eachother. These silica/silica interactions have a detrimental consequenceof limiting the reinforcing properties to a level that is substantiallylower than that which it would be theoretically possible to expect ifall the silica/elastomer interactions capable of being created duringthe mixing operation were actually obtained (this theoretical number ofsilica/elastomer interactions being, as is well known, directlyproportional to the external surface of the silica employed).

Furthermore, in the raw state, such silica/silica interactions tend toincrease the stiffness and the consistency of the mixtures, thus makingthem more difficult to process.

The problem arises of having available fillers which, while beingcapable of being relatively large in size, improve the rheologicalproperties of elastomers and advantageously have a good dispersibilityin elastomers.

The aim of the present invention is to overcome the abovementioneddisadvantages and to solve the abovementioned problem.

More precisely, its aim is especially to propose a new process for thepreparation of precipitated silica which, advantageously, has a gooddispersibility (and disintegratability) and very satisfactoryreinforcing properties, in particular which, when employed as areinforcing filler for elastomers, imparts excellent rheologicalproperties to the latter while providing them with good mechanicalproperties.

The invention also relates to precipitated silicas which, preferably,are in the form of powder, of substantially spherical beads or,optionally, of granules, and which, while being of relatively largesize, have very satisfactory reinforcing properties and, in anadvantageous manner, very good dispersibility (and disintegratability).

It relates, finally, to the use of the said precipitated silicas asreinforcing fillers for elastomers.

In the description which follows, the BET specific surface is determinedaccording to the Brunauer-Emmett-Teller method described in the Journalof the American Chemical Society, Vol. 60, page 309, February 1938 andcorresponding to NFT standard 45007 (November 1987).

The CTAB specific surface is the outer surface determined according toNFT standard 45007 (November 1987) (5.12).

The DOP oil uptake is determined according to NFT standard 30-022 (March1953) by using dioctyl phthalate.

The packing density (PD) is measured according to NFT standard 030100.

The pH is measured according to ISO standard 787/9 (pH of a suspensionat a concentration of 5% in water).

Finally, it is specified that the given pore volumes are measured bymercury porosimetry, the pore diameters being calculated from theWashburn relationship with an angle of contact theta equal to 130° and asurface tension gamma equal to 484 dynes/cm MICROMETRICS 9300POROSIMETER®.

The dispersibility and the disintegratability of the silica according tothe invention can be quantified by means of a specific disintegrationtest.

The disintegration test is carried out according to the followingprocedure:

the cohesion of the agglomerates is assessed by a particle sizemeasurement (using laser scattering), performed on a silica suspensionpreviously disintegrated by ultrasonic treatment; the disintegratabilityof the silica is thus measured (rupture of objects from 0.1 to a fewtens of microns). The disintegration under ultrasound is performed withthe aid of a VIBRACELL BIOBLOCK (600 W)® sonic transducer equipped witha probe 19 mm in diameter. The particle size measurement is performed bylaser scattering on a Sympatec particle size analyser.

2 grams of silica are measured out into a specimen tube (height: 6 cmand diameter: 4 cm) and are made up to 50 grams by adding demineralizedwater; an aqueous suspension containing 4% of silica is thus produced,which is homogenized for 2 minutes by magnetic stirring. Thedisintegration under ultrasound is next performed as follows: with theprobe immersed to a depth of 4 cm, the power is adjusted so as to obtaina needle deflection on the power dial indicating 20% (which correspondsto an energy of 120 watts/cm² dissipated by the end of the probe). Thedisintegration is performed for 420 seconds. The particle sizemeasurement is then carried out after a known volume (expressed in ml)of the homogenized suspension has been introduced into the cell of theparticle size analyser.

The value of the median diameter .O slashed.₅₀ which is obtained isproportionally smaller the higher the disintegratability of the silica.The ratio (10×volume of dispersion introduced (in ml))/optical densityof the suspension detected by the particle size analyser (this opticaldensity is of the order of 20) is also determined. This ratio is anindication of the proportion of fines, that is to say of the content ofparticles smaller than 0.1 μm, which are not detected by the particlesize analyser. This ratio, called the ultrasonic disintegration factor(F_(D)) is proportionally higher the higher the disintegratability ofthe silica.

One of the subjects of the invention is a process for the preparation ofprecipitated silica of the type including the reaction of a silicatewith an acidifying agent, whereby a suspension of precipitated silica isobtained, followed by the separation and the drying of this suspension,in which the precipitation is carried out in the following manner

(i) an initial base stock comprising a silicate of alkali metal M, andan electrolyte is formed, the silicate concentration (expressed as SiO₂)in the said initial base stock being lower than 20 g/l,

(ii) the acidifying agent is added to the said base stock until at least5% of the quantity of M₂ O present in the said base stock isneutralized,

(iii) acidifying agent and a silicate of alkali metal M are addedsimultaneously to the reaction mixture, such that the ratio of thequantity of silicate added (expressed as SiO₂)/the quantity of silicatepresent in the initial base stock (expressed as SiO₂), called the degreeof consolidation, is greater than 4 and at most 100,

characterized in that the said process includes one of the following twooperations (a) or (b):

(a) at least one zinc compound and then a basic agent are added to thereaction mixture after stage (iii) and, when the said separationcomprises a filtration and a disintegration of the cake originating fromthis filtration, the said disintegration is preferably performed in thepresence of at least one aluminium compound,

(b) a silicate and at least one zinc compound are added simultaneouslyto the reaction mixture after stage (iii) and, when the said separationcomprises a filtration and a disintegration of the cake originating fromthis filtration, the disintegration is preferably performed in thepresence of at least one aluminium compound.

It has thus been found that the introduction of zinc--this beingaccording to a particular method--combined with a low silicateconcentration (expressed as SiO₂) in the initial base stock and with anappropriate degree of consolidation during the simultaneous additionstage constitutes an important condition for imparting their goodproperties to the products obtained, especially very satisfactoryreinforcing properties (in particular in respect of the rheology of theelastomers).

It should be noted, in general, that the process concerned is a processfor the synthesis of precipitated silica, that is to say that anacidifying agent is reacted with a silicate in very special conditions.

The choice of the acidifying agent and of the silicate is made in amanner which is well known per se.

It may be recalled that the acidifying agent generally employed is astrong inorganic acid such as sulphuric acid, nitric acid orhydrochloric acid, or an organic acid such as acetic acid, formic acidor carbonic acid.

The acidifying agent may be dilute or concentrated; its normality may bebetween 0.4 and 36 N, for example between 0.6 and 1.5 N.

In particular, in the case where the acidifying agent is sulphuric acid,its concentration may be between 40 and 180 g/l, for example between 60and 130 g/l.

It is possible, furthermore, to employ as a silicate any common form ofsilicates such as metasilicates, disilicates and advantageously analkali metal silicate, especially sodium or potassium silicate.

The silicate may exhibit a concentration, expressed as silica, ofbetween 40 and 330 g/l, for example between 60 and 300 g/l, inparticular between 60 and 250 g/l.

In general, sulphuric acid is employed as the acidifying agent, andsodium silicate as the silicate.

In the case where sodium silicate is employed, the latter generallyexhibits an SiO₂ /Na₂ O weight ratio of between 2 and 4, for examplebetween 3.0 and 3.7.

Insofar as the process of preparation of the invention is moreparticularly concerned, the precipitation is done in a specific manneraccording to the following stages.

First of all a base stock is formed which includes some silicate (stage(i)). The quantity of silicate present in the initial base stockadvantageously represents only a part of the total quantity of silicateintroduced into the reaction.

According to a characteristic of the process of preparation according tothe invention, the silicate concentration in the initial base stock is(higher than 0 g/l and) lower than 20 g of SiO₂ per liter. Thisconcentration may be at most 11 g/l and, optionally, at most 8 g/l.

In particular when the separation performed subsequently during theprocess according to the invention includes a filtration performed bymeans of a filter press (and more particularly in the case where it isdesired to prepare silicas which have a CTAB specific surface of atleast 140 m² /g), this concentration may be at least 8 g/l, inparticular between 10 and 15 g/l, for example between 11 and 15 g/l; thedrying applied later in the process according to the invention may beperformed by spraying by means of a multinozzle sprayer.

The base stock may include an electrolyte. Nevertheless, preferably, noelectrolyte is employed in the course of the process of preparationaccording to the invention; in particular, the initial base stockpreferably does not include any electrolyte.

The term electrolyte is intended to be understood here in its normalaccepted meaning, that is to say that it means any ionic or molecularsubstance which, when it is in solution, decomposes or dissociates toform ions or charged particles. A salt from the group of the alkalimetal and alkaline-earth metal salts may be mentioned as an electrolyte,especially the salt of the metal of the starting silicate and of theacidifying agent, for example sodium sulphate in the case of thereaction of a sodium silicate with sulphuric acid.

The second stage consists in adding the acidifying agent to the basestock of composition described above (stage (ii)).

Thus, in this second stage, the acidifying agent is added to the saidinitial base stock until at least 5%, preferably at least 50%, of the M₂O quantity present in the said initial base stock is neutralized.

In this second stage the acidifying agent is preferably added to theinitial base stock until 50 to 99% of the quantity of M₂ O present inthe said initial base stock is neutralized.

Once the desired value of neutralized M₂ O quantity is reached, then asimultaneous addition (stage (iii)) of acidifying agent and of aquantity of silicate of alkali metal M is undertaken, such that thedegree of consolidation, that is to say the ratio of the quantity ofsilicate added (expressed as SiO₂)/the quantity of silicate present inthe initial base stock (expressed as SiO₂) is higher than 4 and at most100.

According to an alternative form of the process of the invention, thissimultaneous addition of acidifying agent and of a quantity of silicateof alkali metal M is undertaken such that the degree of consolidation ismore particularly between 12 and 100, preferably between 12 and 50,especially between 13 and 40.

According to another alternative form of the process of the inventionthis simultaneous addition of acidifying agent and of a quantity ofsilicate of alkali metal M is undertaken such that the degree ofconsolidation is rather higher than 4 and lower than 12, preferablybetween 5 and 11.5, especially between 7.5 and 11. This alternative formis, in general, used when the silicate concentration in the initial basestock is at least 8 g/l, in particular between 10 and 15 g/l, forexample between 11 and 15 g/l.

Throughout the stage (iii) the quantity of acidifying agent which isadded is preferably such that 80 to 99%, for example 85 to 97%, of thequantity of M₂ O which is added is neutralized.

In stage (iii) it is possible to undertake the simultaneous addition ofacidifying agent and of silicate at a first pH plateau of the reactionmixture, pH₁, and then at a second pH plateau of the reaction mixture,pH₂, such that 7<pH₂ <pH₁ <9.

According to an essential characteristic of the process of preparationaccording to the invention, the latter includes one of the twooperations, (a) or (b) mentioned above, that is to say:

(a) at least one zinc compound and then a basic agent are added, and,when the separation used in the process comprises a filtration and adisintegration of the cake originating from this filtration, the saiddisintegration is preferably performed in the presence of at least onealuminium compound, or

(b) a silicate and at least one zinc compound are added simultaneously,after stage (iii), to the reaction mixture and, when the separation usedin the process comprises a filtration, the disintegration of the cakeoriginating from this filtration is preferably performed in the presenceof at least one aluminium compound.

In a first alternative form of the process of preparation according tothe invention (that is to say when the latter includes the operation(a)), the following successive stages are performed advantageously afterhaving carried out the precipitation according to the stages (i), (ii)and (iii) described above:

(iv) at least one zinc compound is added to the reaction mixture (thatis to say to the reaction suspension or slurry obtained),

(v) a basic agent is added to the reaction mixture preferably until a pHvalue of the reaction mixture of between 7.4 and 10, in particularbetween 7.8 and 9, is obtained,

(vi) acidifying agent is added to the reaction mixture, preferably untila pH value of the reaction mixture of at least 7, in particular between7 and 8.5, for example between 7 and 8, is obtained.

After the simultaneous addition of stage (iii) it may then beadvantageous to perform a maturing of the reaction mixture, it beingpossible for this maturing to last, for example, from 1 to 60 minutes,in particular from 3 to 30 minutes.

In this first alternative form it is desirable, between stage (iii) andstage (iv), and especially before the said optional maturing, to add anadditional quantity of acidifying agent to the reaction mixture. Thisaddition is generally done until a pH value of the reaction mixture ofbetween 3 and 6.5, in particular between 4 and 6, is obtained.

The acidifying agent employed during this addition is generallyidentical with that employed during stages (ii), (iii) and (vi) of thefirst alternative form of the process of preparation according to theinvention.

A maturing of the reaction mixture is usually performed between stage(v) and (vi), for example for 2 to 60 minutes, in particular for 5 to 45minutes.

Similarly, a maturing of the reaction mixture is in most cases performedafter stage (vi), for example for 2 to 60 minutes, in particular for 5to 30 minutes.

The basic agent employed during stage (iv) may be a solution of aqueousammonia or, preferably, a solution of sodium hydroxide (or soda).

In a second alternative form of the process of preparation according tothe invention (that is to say when the latter includes the operation(b)), a stage (iv) is performed after the stages (i), (ii) and (iii)described previously, which consists in adding a silicate and at leastone zinc compound simultaneously to the reaction mixture.

After the simultaneous addition of stage (iv) it may then beadvantageous to perform a maturing of the reaction mixture, it beingpossible for this maturing to last, for example, from 2 to 60 minutes,in particular from 5 to 30 minutes.

In this second alternative form it is desirable, after stage (iv), andespecially after this optional maturing, to add an additional quantityof acidifying agent to the reaction mixture. This addition is generallydone until a pH value of the reaction mixture of at least 7, inparticular between 7 and 8.5, for example between 7 and 8, is obtained.

The acidifying agent employed during this addition is generallyidentical with that employed during stages (ii) and (iii) of the secondalternative form of the process of preparation according to theinvention.

A maturing of the reaction mixture is usually performed after thisaddition of acidifying agent, for example for 1 to 60 minutes, inparticular for 3 to 30 minutes.

The zinc compound employed in the process of preparation according tothe invention is generally an organic or inorganic zinc salt.

By way of examples of an organic salt there may be mentioned especiallythe salts of carboxylic or polycarboxylic acids, like the salts ofacetic, citric, tartaric or oxalic acid.

By way of examples of an inorganic salt there may be mentionedespecially halides and oxyhalides (like chlorides and oxychlorides),nitrates, phosphates, sulphates and oxysulphates.

In practice, the zinc compound may be employed in the form of asolution, generally aqueous.

A zinc sulphate is preferably employed as zinc compound.

The temperature of the reaction mixture is generally between 60 and 98°C.

According to an alternative form of the invention the reaction isperformed at a constant temperature of between 70 and 96° C.

According to another alternative form of the invention the temperatureat the end of the reaction is higher than the temperature at thebeginning of reaction; the temperature at the beginning of the reactionis thus maintained preferably between 70 and 96° C. and the temperatureis then raised over a few minutes, preferably up to a value of between75 and 98° C., which value it is maintained until the end of thereaction; the operations (a) or (b) are thus usually performed at thisconstant temperature value.

At the outcome of the stages which have just been described a silicaslurry is obtained which is then separated (liquid-solid separation).

In the first alternative form of the process of the preparationaccording to the invention (that is to say when the latter includes theoperation (a)), this separation comprises, in general, a filtration(followed by washing if necessary) and a disintegration, the saiddisintegration being performed in the presence of at least one zinccompound and, preferably, in the presence of an acidifying agent asdescribed above (in this latter case the zinc compound and theacidifying agent are advantageously added simultaneously).

The disintegration operation, which may be carried out, for example, bypassing the filter cake through a mill of the colloid or bead type,makes it possible in particular to lower the viscosity of the suspensionto be subsequently dried.

In the second alternative form of the process of preparation accordingto the invention (that is to say when the latter includes the operation(b)), the separation also comprises, in general, a filtration (followedby washing if necessary) and a disintegration, the said disintegrationbeing preferably performed in the presence of at least one aluminiumcompound and, in general, in the presence of an acidifying agent asdescribed above (in this latter case the aluminium compound and theacidifying agent are advantageously added simultaneously).

The aluminium compound generally consists of an alkali metal, especiallypotassium, or, very preferably, sodium, aluminate.

The quantity of the zinc compound employed in the process of preparationaccording to the invention is preferably such that the precipitatedsilica prepared contains more between 1 and 5%, in particular between1.5 and 4%, for example between 1.5 and 2.5%, by weight of zinc.

The separation used in the process of preparation according to theinvention usually includes a filtration performed by means of anysuitable method, for example by means of a belt filter, a rotary vacuumfilter or, preferably, a filter press.

The suspension of precipitated silica thus recovered (filter cake) isthen dried.

This drying may be done according to any method that is known per se.

The drying is preferably done by spraying.

Any suitable type of sprayer may be employed for this purpose,especially a turbine, nozzle, liquid-pressure or two-fluid sprayer.

The drying is, for example, performed by spraying by means of amultinozzle sprayer especially when the concentration of silicate in theinitial base stock is at least 8 g/l (and lower than 20 g/l), inparticular between 10 and 15 g/l (and more particularly in the case whenit is desired to prepare silicas which have a CTAB specific surface ofat least 140 m² /g).

According to one embodiment of the invention, the suspension to be driedhas a solids content higher than 15% by weight, preferably higher than17% by weight and, for example, higher than 20% by weight. The drying isthen preferably performed by means of a multinozzle sprayer.

The precipitated silica capable of being obtained according to thisembodiment of the invention and preferably by using a filter press isadvantageously in the form of substantially spherical beads, preferablyof a mean size of at least 80 μm.

It should be noted that dry material for example silica in pulverulentform may be also added to the filter cake after the filtration, at asubsequent stage of the process.

At the end of the drying, a stage of milling may be undertaken on theproduct recovered, especially on the product obtained by drying asuspension which has a solids content higher than 15% by weight Theprecipitated silica which is then obtainable is generally in the form ofa powder, preferably with a mean size of at least 15 μm, in particularbetween 15 and 60 μm, for example between 20 and 45 μm.

The milled products with the desired particle size can be separated fromany nonconforming products by means, for example, of vibrating screenswhich have appropriate mesh sizes, and the nonconforming products thusrecovered can be returned to the milling.

Similarly, according to another embodiment of the invention, thesuspension to be dried has a solids content of at most 15% by weight.The drying is then generally performed by means of a turbine sprayer.The precipitated silica which is then obtainable according to thisembodiment of the invention and preferably by using a rotary vacuumfilter is generally in the form of a powder, preferably with a mean sizeof at least 15 μm, in particular between 30 and 150 μm, for examplebetween 45 and 120 μm.

Finally, the product which has been dried (especially from a suspensionwhich has a solids content of at most 15% by weight) or milled can,according to another embodiment of the invention, be subjected to anagglomeration stage.

Agglomeration is here intended to mean any process which enables finelydivided objects to be bonded together in order to bring them into theform of objects of larger size and which are mechanically stronger.

These processes are especially direct compression, wet-route granulation(that is to say with the use of a binder such as water, silica slurry,etc.), extrusion and, preferably, dry compacting.

When this last technique is used it may be found advantageous, beforestarting the compacting, to deaerate the pulverulent products (anoperation which is also called predensifying or degassing), so as toremove the air included therein and to ensure a more uniform compacting.

The precipitated silica which can be obtained according to thisembodiment of the invention is advantageously in the form of granules,preferably at least 1 mm in size, in particular between 1 and 10 mm.

At the end of the agglomeration stage the products may be classified toa desired size, for example by screening, and then packaged for theirfuture use.

The powders, as well as the beads, of precipitated silica which areobtained by the process according to the invention thus offer theadvantage, among others, of providing access to granules such as thosementioned above, in a simple, efficient and economical manner,especially by conventional forming operations, such as, for example,granulation or compacting, without the latter resulting in degradationcapable of masking, or even annihilating, the good intrinsic propertiesassociated with these powders or these beads, as may be the case in theprior art when using conventional powders.

Other subjects of the invention consist of new precipitated silicaswhich have, in an advantageous manner, a good dispersibility (anddisintegratability) and very satisfactory reinforcing properties, inparticular which, when employed as a reinforcing filler for elastomers,impart very good rheological properties to the latter while providingthem with very satisfactory mechanical properties.

Thus, a new precipitated silica is now proposed, according to theinvention, characterized in that it has:

a CTAB specific surface of between 90 and 250 m² /g, for example between120 and 230 m² /g,

a DOP oil uptake lower than 300 ml/100 g, preferably between 200 and 295ml/100 g,

a pore distribution such that the pore volume consisting of the poreswhose diameter is between 175 and 275 Å represents less than 50% of thepore volume consisting of the pores of diameters which are smaller thanor equal to 400 Å,

a zinc of between 1 and 5% by weight, preferably between 1.5 and 4% byweight,

and in that the number N of molecules of stearic acid consumed per nm²of silica surface, when stearic acid is reacted with the said silica inxylene for 2 hours at 120° C., is at least 1, preferably at least 1.2,in particular at least 1.5.

The silica according to the invention preferably has a zinc content ofbetween 1.5 and 4% by weight; this content may be especially between 1.5and 2.5% by weight.

One of the essential characteristics of the precipitated silicaaccording to the invention is its consumption, in a model medium(xylene), of an ingredient of rubber vulcanization (stearic acid).

The Applicant Company has thus found that the precipitated silicasexhibiting a particular number N, in combination with the othercharacteristics mentioned in the present description, made it possiblein particular to impart very good rheological properties to theelastomers while providing them with satisfactory mechanical properties.

To determine this characteristic (number N), stearic acid is reacted inthe presence of silica in xylene for 2 hours at 120° C. The quantity ofstearic acid remaining in the xylene after reaction is then determinedby infrared (IR) spectrometry; the quantity of stearic acid which hasbeen consumed by the silica can then be deduced and, hence, the number Nof molecules of stearic acid consumed per nm² of the silica surface.

The operating method employed for determining this characteristic isdescribed more precisely below.

60.2 g (that is 70 ml) of xylene are added into a round bottom flaskcontaining 3.17 g of stearic acid. The flask is stoppered and is thenmagnetically stirred for a few minutes.

12.04 g of silica are then added.

The flask is placed in an oil bath at 120° C. under reflux (a condenserbeing fitted to it). The flask is then magnetically stirred for 105 min.The stirring is then stopped and the flask is left in the oil bath foranother 15 min. The total duration of the reaction at 120° C. istherefore 2 hours.

The condenser is removed and the flask is taken out of the oil bath.

The contents of the flask are filtered on a microfiltration system(Millipore unit with Durapore membrane filters made of polyvinylidenefluoride (pore size: 0.45 μm)).

10 g of the filtrate obtained are then diluted in 10 g of xylene; asolution S is obtained.

In parallel, standard solutions of stearic acid in xylene are prepared(which have a stearic acid content lower than 2 mass %) and the IRspectra (from 400 to 4000 cm⁻¹) of each of them are produced. Thecharacteristic peak of stearic acid is situated at 1710 cm⁻¹. Theintensity of this peak associated with the stearic acid content of thesolution makes it possible to plot the straight line of the stearic acidcontent of the solution as a function of the IR absorbance at 1710 cm⁻¹; the equation of the calibration straight line is obtained by linearregression.

Similarly, the IR spectrum of the solution S is produced. The value ofthe characteristic peak of stearic acid, referred to the equation of thecalibration straight line, allows the content of stearic acid present inthe solution S to be determined; by taking into account the mass ofxylene added during the dilution, the stearic acid content of thefiltrate from the reaction is obtained. The content and therefore thequantity of stearic acid consumed by the silica during the reaction arededuced from the initial stearic acid content and from the stearic acidcontent after reaction (the latter being the stearic acid content of thefiltrate). The number N of molecules of stearic acid which are consumedper nm² of the silica surface is then determined.

The zinc present in the precipitated silica according to the inventionis preferably not in crystalline form, but is rather present inamorphous form (this can be determined by X-ray diffraction).

Another characteristic of the silica according to the invention lies inthe distribution, or spread, of the pore volume and especially in thedistribution of the pore volume which is produced by the pores ofdiameters smaller than or equal to 400 Å. This latter volume correspondsto the useful pore volume of the fillers which are employed in thereinforcement of elastomers. Analysis of the porograms shows that thesilica according to the invention then has a pore distribution such thatthe pore volume consisting of the pores whose diameter is between 175and 275 Å represents less than 50%, for example less than 40%, of thepore volume consisting of the pores of diameters smaller than or equalto 400 Å.

According to a first (preferred) particular embodiment of the invention,the precipitated silica has:

a CTAB specific surface of between 90 and 185 m² /g,

a median diameter (.O slashed.₅₀), after disintegration with ultrasound,smaller than 6 μm, preferably smaller than 5 μm.

It then generally has a BET specific surface of between 90 and 230 m²/g, in particular between 100 and 190 m² /g, for example between 120 and190 m² /g.

According to an alternative form of this embodiment of the invention theprecipitated silica has:

a CTAB specific surface of between 90 and 140 m² /g, for example between100 and 135 m² /g, for example between 120 and 135 m² /g,

a median diameter (.O slashed.₅₀), after disintegration with ultrasound,smaller than 4.5 μm, in particular smaller than 4 μm, for examplesmaller than 3.8 μm.

According to another alternative form of this embodiment of theinvention the precipitated silica has:

a CTAB specific surface of between 140 and 185 m² /g,

an ultrasonic disintegration factor (F_(D)) higher than 5.5 ml, inparticular higher than 11 ml.

According to a second particular embodiment of the invention theprecipitated silica has:

a CTAB specific surface higher than 185 m² /g and lower than 220 m² /g,

a median diameter (.O slashed.₅₀), after disintegration with ultrasound,smaller than 8.5 μm, preferably smaller than 7 μm.

It then generally has a BET specific surface of between 190 and 280 m²/g, especially between 190 and 250 m² /g.

The ultrasonic disintegration factor (F_(D)) of the precipitated silicaaccording to this particular embodiment of the invention may be higherthan 5.5 ml.

According to an alternative form of the invention the silica has a BETspecific surface/CTAB specific surface ratio of between 1.0 and 1.2,that is to say that it preferably exhibits very low microporosity.

According to another alternative form of the invention the silica has aBET specific surface/CTAB specific surface ratio higher than 1.2, forexample of between 1.21 and 1.4, that is to say that it exhibits arelatively high microporosity.

The pH of the silica according to the invention is generally between 8.0and 9.0, for example between 8.3 and 8.9.

The silicas according to the invention may be in the form of powders, ofsubstantially spherical beads or, optionally, of granules, and arecharacterized particularly by the fact that, while being relativelylarge in size, they have very satisfactory reinforcing properties and,preferably, a very good dispersibility and disintegratability.

The silica powders according to the invention preferably have a meansize of at least 15 μm; the latter is, for example, between 15 and 60 μm(especially between 20 and 45 μm) or between 30 and 150 μm (especiallybetween 45 and 120 μm).

They have, preferably, a DOP oil uptake of between 240 and 290 ml/100 g.

The packing density (PD) of the said powders is generally at least 0.17and, for example, between 0.2 and 0.3.

The said powders generally have a total pore volume of at least 2.5 cm³/g and, more particularly, of between 3 and 5 cm³ /g.

They make it possible in particular to obtain a very good compromisebetween processing and mechanical properties in the vulcanized state.

They also constitute preferred precursors for the synthesis ofgranulates as described later.

The substantially spherical beads according to the invention preferablyhave a mean size of at least 80 μm.

According to certain alternative forms of the invention, this mean beadsize is at least 100 μm, for example at least 150 μm; it is generally atmost 300 μm and preferably lies between 100 and 270 μm. This mean sizeis determined according to NF standard×11507 (December 1970) by dryscreening and determination of the diameter corresponding to acumulative oversize of 50%.

They preferably have a DOP oil uptake of between 240 and 290 ml/100 g.

The packing density (PD) of the said beads (or prills) is generally atleast 0.17 and, for example, between 0.2 and 0.34.

They usually have a total pore volume of at least 2.5 cm^(3/) g and,more particularly, of between 3 and 5 cm³ /g.

As indicated above, such a silica in the form of substantially sphericalbeads which are advantageously solid, homogeneous and low in dust andhave good pourability, has a good disintegratability and dispersibility.In addition, it exhibits good reinforcing properties. Such a silica alsoconstitutes a preferred precursor for the synthesis of the powders andgranules according to the invention.

Such a silica in the form of substantially spherical beads constitutes ahighly advantageous alternative form of the invention.

The dimensions of the granules according to the invention are preferablyat least 1 mm, in particular between 1 and 10 mm, along the axis oftheir largest dimension (length).

They preferably have a DOP oil uptake of between 200 and 260 ml/100 g.

The said granules may be of the most diverse shape. The shapes which maybe especially mentioned by way of example are the spherical,cylindrical, parallelepipedal, tablet, flake, pellet and extrudate ofcircular or polylobar section.

The packing density (PD) of the said granules is generally at least 0.27and may range up to 0.37.

They generally have a total pore volume of at least 1 cm³ /g and, moreparticularly, between 1.5 and 2 cm³ /g.

The silicas according to the invention, especially in the form of powderor of substantially spherical beads or granules are preferably preparedaccording to one of the appropriate alternative forms of the process ofpreparation in accordance with the invention and described above.

The silicas according to the invention or prepared by the processaccording to the invention find a particularly advantageous applicationin the reinforcement of natural or synthetic elastomers. They impartexcellent rheological properties to these elastomers while providingthem with good mechanical properties and, in general, good resistance toabrasion. In addition, these elastomers are preferably less liable toheating.

The invention therefore also relates to the use of these silicas forimproving the Theological properties of elastomers.

The following examples illustrate the invention without, however,limiting its scope.

EXAMPLE 1

The following were introduced into a stainless steel reactor providedwith a stirring system using propellers and with heating using a jacket:

733 liters of water, and

46.5 liters of a solution of sodium silicate (SiO₂ /Na₂ O weight ratioof 3.4) with a concentration expressed as silica of 235 g/l.

The concentration of silicate expressed as SiO₂ in the initial basestock is thus 14 g/l. The temperature of the solution was then raised to80° C. while being kept stirred. The entire reaction was carried out at80° C. and with stirring. Dilute sulphuric acid with a density of 1.050at 20° C. was then introduced at a rate of 5.4 l/min for a period of 9minutes; following this addition, the neutralisation ratio in the basestock was 78%, i.e., 78% of the quantity of Na₂ O present in the initialbase stock had been neutralised.

Simultaneous introduction of a sodium silicate solution of the typedescribed above at a rate of 4.3 l/min and of dilute sulphuric acid alsoof the type described above and at a rate which was regulated sa as tomaintain a pH:

of 8.5±0.1 for the first 55 minutes, then

of 7.8±0.1 for the final 35 minutes,

in reaction medium, was then effected over 90 minutes.

During this simultaneous addition step, the instantaneous neutralisationratio was 94%, i.e., 94% of the quantity of Na₂ O added (per min) wasneutralised.

The consolidation ratio following simultaneous addition was 8.3.

After this simultaneous addition an aqueous solution containing 85 g/lof zinc sulphate is next introduced in the reaction medium for 12minutes at a flow rate of 9.3 l/min. At the end of this addition anaqueous solution containing 180 g/l of sodium hydroxide is introducedinto the reaction medium until the pH of the reaction mixture is equalto 8.9.

The introduction of sodium hydroxide is then stopped and the reactionmedium is kept stirred for 10 minutes.

Sulphuric acid of the type described above is then introduced until thepH of the reaction medium is equal to 7.1.

The introduction of acid is then stopped and maturing of the reactionmedium is undertaken for 5 minutes at at temperature of 80° C.

The total reaction period is 148 minutes.

A slurry or suspension of precipitated silica is thus obtained, which isnext filtered and washed by means of a filter press.

The cake obtained is next fluidized by mechanical and chemical action(simultaneous addition of sulphuric acid and of a quantity of sodiumaluminate corresponding to an Al/SiO₂ weight ratio of 0.30%). After thisdisintegration operation, the resulting slurry, with a pH equal to 8.4and a loss on ignition equal to 78.0% (and therefore a solids content of22.0% by weight), is sprayed by means of a nozzle sprayer.

The characteristics of the silica P1 obtained in the form ofsubstantially spherical beads are as follows:

    ______________________________________                                        CTAB specific surface 145    m.sup.2 /g                                         BET specific surface 175 m.sup.2 /g                                           DOP oil uptake 275 ml/100 g                                                   Zinc weight content 1.80 %                                                    Pore volume V1 represented 0.95 cm.sup.3 /g                                   by the pores of d ≦ 400 Å                                          Pore volume V2 represented 0.40 cm.sup.3 /g                                   by the pores 175 Å ≦ d ≦ 275 Å                          V2/V1 ratio 42 %                                                              pH 8.5                                                                        Mean particle size 210 μm                                                ______________________________________                                    

The number N of stearic acid molecules consumed per nm² of silicasurface, when stearic acid is reacted with the said silica P1 in xylenefor 2 hours at 120° C. (in accordance with the operating procedureoutlined in the description), is equal to 1.4.

The silica P1 is subjected to the disintegration test as defined abovein the description.

After disintegration with ultrasound it has a median diameter (.Oslashed.₅₀) of 2.7 μm and an ultrasonic disintegration factor (F_(D)) of16 ml.

EXAMPLE 2

The following were introduced into a stainless steel reactor providedwith a stirring system using propellers and with heating using a jacket:

626 liters of water, and

36 liters of a solution of sodium silicate (SiO₂ /Na₂ O weight ratio of3.4) with a concentration expressed as silica of 130 g/l.

The concentration of silicate expressed as SiO₂ in the initial basestock is thus 7.1 g/l. The temperature of the solution was then raisedto 95° C. while being kept stirred. The entire reaction was carried outat 95° C. and with stirring. A sulphuric acid with a concentration of 80g/l was then introduced at a rate of 5.4 l/min for a period of 3 minutesand 20 seconds; following this addition, the neutralisation ratio in thebase stock was 67%, i.e., 67% of the quantity of Na₂ O present in theinitial base stock had been neutralised.

Simultaneous introduction in the reaction medium of:

a sulphuric acid solution of the type described above at a rate of 5.4l/min, and

a sodium silicate solution of the type described above at a rate of 9.2l/min was then effected over 70 minutes.

During this simultaneous addition step, the instantaneous neutralisationratio was 79%, i.e., 79% of the quantity of Na₂ O added (per min) wasneutralised.

The consolidation ratio following simultaneous addition was 17.9.

After this simultaneous addition an aqueous solution containing 85 g/lof zinc sulphate is next introduced in the reaction medium for 12minutes at a flow rate of 9.3 l/min. At the end of this addition anaqueous solution containing 180 g/l of sodium hydroxide is introducedinto the reaction medium until the pH of the reaction mixture is equalto 8.9.

The introduction of sodium hydroxide is then stopped and the reactionmedium is kept stirred for 10 minutes.

Sulphuric acid of the type described above is then introduced until thepH of the reaction medium is equal to 7.1.

The introduction of acid is then stopped and maturing of the reactionmedium is undertaken for 5 minutes at at temperature of 95° C.

The total reaction period is 127 minutes.

A slurry or suspension of precipitated silica is thus obtained, which isnext filtered and washed by means of a filter press.

The cake obtained is next fluidized by mechanical and chemical action(simultaneous addition of sulphuric acid and of a quantity of sodiumaluminate corresponding to an Al/SiO₂ weight ratio of 0.20%). After thisdisintegration operation, the resulting slurry, with a pH equal to 8.4and a loss on ignition equal to 79.0% (and therefore a solids content of21.0% by weight), is sprayed by means of a nozzle sprayer.

The characteristics of the silica P2 obtained in the form ofsubstantially spherical beads are as follows:

    ______________________________________                                        CTAB specific surface 135    m.sup.2 /g                                         BET specific surface 147 m.sup.2 /g                                           DOP oil uptake 250 ml/100 g                                                   Zinc weight content 1.90 %                                                    Pore volume V1 represented 0.86 cm.sup.3 /g                                   by the pores of d ≦ 400 Å                                          Pore volume V2 represented 0.39 cm.sup.3 /g                                   by the pores 175 Å ≦ d ≦ 275 Å                          V2/V1 ratio 45 %                                                              pH 8.5                                                                        Mean particle size 220 μm                                                ______________________________________                                    

The number N of stearic acid molecules consumed per nm² of silicasurface, when stearic acid is reacted with the said silica P2 in xylenefor 2 hours at 120° C. (in accordance with the operating procedureoutlined in the description), is equal to 1.7.

The silica P2 is subjected to the disintegration test as defined abovein the description.

After disintegration with ultrasound it has a median diameter (.Oslashed.₅₀) of 3.2 μm and an ultrasonic disintegration factor (F_(D)) of14.5 ml.

The characteristics of the silicas prepared in Examples 1 and 2 andthose of a commercial silica sold in the form of substantially sphericalbeads by RHONE-POULENC CHIMIE as a reinforcing filler for elastomers, inthis case the silica ZEOSYL® 175 MP (referred to as MP1), are reportedin Table 1 below.

                  TABLE I                                                         ______________________________________                                                   MP1        P1     P2                                               ______________________________________                                        S.sub.CTAB (m.sup.2 /g)                                                                    162          145    135                                            S.sub.BET (m.sup.2 /g) 175 175 147                                            DOP (ml/100 g) 280 275 250                                                    Zn (%) <0.005 1.80 1.90                                                       V1 (cm.sup.3 /g) 0.95 0.95 0.86                                               V2 (cm.sup.3 /g) 0.45 0.40 0.39                                               V2/V1 (%) 47 42 45                                                            pH 6.5 8.5 8.5                                                                Mean size 265 210 220                                                         (μm)                                                                       N (mol/nm.sup.2) 0.5 1.4 1.7                                                  .O slashed..sub.50 (μm) 9.1 2.7 3.2                                        F.sub.D (ml) 2.1 16 14.5                                                    ______________________________________                                    

EXEMPLE 3

This example illustrates the use and the behaviour of a silica accordingto the invention and of a silica not in accordance with the invention ina formulation for industrial rubber.

The following formulation is employed (the parts are expressed byweight):

    ______________________________________                                        S.B.R. 1955 S 25 rubber.sup.(1)                                                                  50                                                           B.R. 1220 rubber.sup.(2)                                                      Natural rubber 25                                                             Silica 51                                                                     Active ZnO.sup.(3) 1.8                                                        Stearic acid 0.35                                                             6PPD.sup.(4) 1.45                                                             CBS.sup.(5) 1.1                                                               DPG.sup.(6) 1.4                                                               Sulphur.sup.(7) 0.9                                                           Silane X50S.sup.(8) 8.15                                                    ______________________________________                                         .sup.(1) Styrene butadiene copolymer solution type 1955 S 25                  .sup.(2) Butadiene polymer type 1220                                          .sup.(3) Rubber grade zinc oxide                                              .sup.(4) N(1,3-Dimethylbutyl)-Nphenyl-p-phenylenediamine                      .sup.(5) NCyclohexyl 2benzothiazyl sulphenamide                               .sup.(6) Diphenyl guanidine                                                   .sup.(7) Vulcanizing agent                                                    .sup.(8) Silica/rubber coupling agent (product marketed by Degussa)      

The formulations are prepared in the following manner:

The following are introduced into an internal mixer (Banbury type), inthis order and at the times and temperatures of the mixing which areshown in brackets:

S.B.R. 1955 S 25, B.R. 1220 and natural rubber (t₀)(60° C.)

the X50S and 2/3 of the silica (t₀ +1 min)(80° C.)

the ZnO, the stearic acid, the 6 PPD and 1/3 of the silica (t₀ +2min)(100° C.)

The discharge from the mixer (mix drop) takes place when the chambertemperature reaches 165° C. (that is to say at approximately t₀ +5 min10 s). The mix is introduced onto a roll mill, the rolls being kept at30° C., to be calendered thereon. The CBS, the DPG and the sulphur areintroduced onto this mill.

After homogenization and three fine passes the final mixture iscalendered into the form of sheets from 2.5 to 3 mm in thickness.

The results of the tests are the following:

1--Rheological properties

The measurements are carried out on the formulations in the raw state.

The results are reported in Table II below. The apparatus employed forconducting the measurements has been shown.

                  TABLE II                                                        ______________________________________                                                           MPI  P1                                                    ______________________________________                                        MOONEY consistency .sup.(1)                                                                        130    85                                                  Min. torque (In.lb) .sup.(2) 26.5 19.1                                      ______________________________________                                         .sup.(1) MOONEY MV 2000E viscometer (Mooney large (1 + 4) measurement at      100° C.)                                                               .sup.(2) MONSANTO 100 S rheometer                                        

The formulation obtained from the silica according to the inventionresults in the lowest values.

This expresses a greater processibility of the mixes prepared from thesilica according to the invention, in particular in respect of theextrusion and calendering operations which are often carried out duringthe manufacture of elastomer compositions (lower energy expenditure forprocessing the mix, greater ease of injection during the compounding,less die swell during extrusion, less shrinkage on calendering, . . . ).

2--Mechanical properties

The measurements are carried out on vulcanized formultaions.

The vulcanization is carried out by heating the formulations to 150° C.for 40 minutes.

The following standards were employed:

(i) tensile tests (moduli, tensile strength) NFT 466002 or ISO 37-1977

(ii) tests of abrasion resistance DIN 53-516

The results obtained are listed in Table III below.

                  TABLE III                                                       ______________________________________                                                           MP1  P1                                                    ______________________________________                                        300% modulus/100% modulus                                                                          3.4    5.4                                                 Tensile strength (MPa) 17.1 20.8                                              Abrasion resistance (mm.sup.3) .sup.(1) 58 54                               ______________________________________                                         .sup.(1) the measured value is the loss on abrasion: the lower it is, the     better the abrasion resistance.                                          

These last results demonstrate the good reinforcing effect conferred bythe silica according to the invention.

Thus, while resulting in more satisfactory Theological properties, thesilica according to the invention provides mechanical properties whichare better than those obtained with the silica of the prior art.

On the one hand, the silica according to the invention produces a 300%modulus/100% modulus ratio that is greater than the ratio obtained withthe silica of the prior art, which is a proof of better dispersion ofthe silica within the rubber matrix.

On the other hand, the high reinforcing power of the silica according tothe invention is confirmed by the high value obtained for the tensilestrength.

At last, with regard to the abrasion resistance, it can be seen that theloss by abrasion is reduced in relation to the comparative silica.

3--Dynamic properties

The measurements are carried out on vulcanized formulations.

The vulcanization is obtained by heating the formulations to 150° C. for40 minutes. The results (illustrating the tendency to heat up) arereported in Table IV below (the lower the value, the lower the tendencyto heat up). The apparatus employed for conducting the measurements hasbeen shown.

                  TABLE IV                                                        ______________________________________                                                             MP1    P1                                                ______________________________________                                          Internal heating (°C.).sup.(1) 90 69                                 ______________________________________                                         .sup.(1) GOOORICH flexometer                                             

The tendency to heat up obtained from the silica according to theinvention is low.

What is claimed is:
 1. A precipitated silica having:a CTAB specificsurface of between 90 and 250 m² /g, a DOP oil uptake lower than 300ml/100 g, a pore distribution and a pore volume such that the porevolume consisting of the pores whose diameter is between 175 and 275 Årepresents less than 50% of the pore volume consisting of the pores ofdiameters which are smaller than or equal to 400 Å, and a zinc contentof between 1 and 5% by weight, said silica being in the form selectedfrom the group consisting of spherical beads with a mean size of atleast 80 μm, a powder with a mean size of at least 15 μm, and granulesof at least 1 mm in size.
 2. A silica according to claim 1, wherein thezinc is not in crystalline form.
 3. A silica according to claim 1,wherein the zinc content is between 1.5 and 4% by weight.
 4. A silicaaccording to claim 1, wherein the CTAB specific surface is between 90and 185 m² /g.
 5. A silica according to claim 1, further having a mediandiameter (.O slashed.₅₀), after disintegration with ultrasound, smallerthan 8.5 μm.
 6. A silica according to claim 5, wherein the CTAB specificsurface is between 90 and 140 m² /g, and the median diameter (.Oslashed.₅₀), after disintegration with ultrasound, is smaller than 4.5μm.
 7. A silica according to claim 1, wherein the CTAB specific surfaceis between 140 and 185 m² /g.
 8. A silica according to claim 1, furtherhaving an ultrasonic disintegration factor (F_(D)) higher than 5.5 ml.9. A silica according to claim 5, wherein the CTAB specific surface ishigher than 185 m² /g and lower than 220 m² /g.
 10. A silica accordingto claim 9, wherein it has an ultrasonic disintegration factor (F_(D))higher than 5.5 ml.
 11. A silica according to claim 1, wherein saidsilica is in the form of spherical beads and wherein the mean size ofsaid spherical beads is at least 150 μm.
 12. An elastomer comprising, asreinforcing filler, a silica according to claim 1.